2.0 Analysis 2.1 General The action of the pilot to turn around and proceed toward the fire road indicates that he likely detected a problem and was returning to the staging site, but did not have time to reach the road. Given the flight path, it is likely that the engine failure began as a rapid and progressive deterioration prior to the ultimate failure of the compressor assembly. The combination of altitude and terrain features placed the helicopter in a flight regime from which the probability of a successful autorotation was minimal. The snagging of the bucket and then the trailing longline on a tree had a further negative effect on the pilot's ability to complete a successful emergency landing. The pilot's reaction to these events may provide an explanation for the loss of rotor rpm. As neither the helicopter's performance nor the drive train was considered to be a factor in the accident, this analysis focuses on the engine power loss and the reason the longline remained attached to the helicopter. 2.2 Engine Power Loss 2.2.1 General The results of the engine teardown indicated that the following two events occurred within the gas generator section: excessive temperature at the compressor turbine, and contact between the compressor rotor and stator assemblies. 2.2.2 Excessive Turbine Temperature The pilot report that the N2was drooping indicated that the engine was reaching the maximum fuel flow allowed through the N1take-off trim setting. Subsequently, the N1take-off trim on the FCU was further increased by a small amount. Since the weight of the helicopter with the loaded bucket exceeded the maximum hover-out-of-ground-effect weight for the conditions, effective piloting technique in combination with as much engine power as possible would have been required to lift the loaded bucket out of the lake. While a higher-than-permitted FCU take-off trim setting would not cause the gas generator to exceed the allowable limit, it also would not provide protection against an overspeed12 due to other reasons, such as a high power demand. Because the helicopter was lifting better after the final adjustment to the N1take-off trim and the trim was adjusted at least twice since engine installation in the helicopter, it is possible that the engine was then being operated above the N1 placarded limit. If the adjustment was too high, it is a possibility that the engine was operating within the correct EGT range, but in the range where internal engine temperatures were unknowingly high, resulting in an increased risk of degradation of the hot section components. Since the PT blades and even the PTnozzle, immediately behind the CT, did not receive the thermal damage exhibited by the CT blades, it can be concluded that a surge in temperature occurred, but the excessive temperature was not sustained long enough to cause melting of the PT nozzle and blades before the engine flamed out. Since no N1topping adjustments were made for about nine hours of operation prior to the accident, it is unlikely that an excessive N1trim setting could have produced the sudden surge in temperature exhibited. However, when the degradation of the compressor resulted in the PTG increasing the fuel flow, any damage occurring due to an excessive temperature condition would likely have been accelerated by a higher-than-normal fuel flow. 2.2.3 Compressor Rotor Contact The TSB Engineering Laboratory was tasked to explain the elliptical shapes of the compressor blade tip plots (see section1.5.2, paragraph5) and, in summary, provided the following: The components that make up the rotor assembly all have a circular shape and are designed to rotate about their geometric centre (axis of rotation). A critical design element of any high speed rotating assembly is the balancing of the individual components and of the complete assembly. In a balanced condition, the rotor's centre of mass (mass centreline) is the same as the rotor's axis of rotation. An unbalanced condition is caused by the displacement of the mass centreline from the axis of rotation through the addition or loss of material (mass). This imbalance results in centrifugal forces that cause the assembly to wobble and may result in contact with the stationary component. The results of this contact could be as minimal as blade tip rub (smearing of the blade tip) or as extensive as complete destruction. Since the rotor was dynamically balanced and successful vibration checks were carried out on the engine at the overhaul facility, and again after installation into the helicopter, it is concluded that the rotor assembly's mass centreline was the same as its axis of rotation. The elliptical shape of the blade tip plots was due to the differences in lengths of the blades and likely due to a subsequent misalignment in the rotor assembly. The damage found within the compressor rotor could only have occurred during the overhaul, and there were known deviations from the specified overhaul procedure. Therefore, the Engineering Laboratory also examined the rotor components to determine whether incomplete seating during assembly may have resulted in seating taking place during operation, causing a loss of torque on the retainer bolts and allowing the assembly to flex and wobble. The Engineering Laboratory reported that: . . . if the screws had subsequently lost their torque during engine operation, then the rotor components would no longer be held in compression, and it would be reasonable to expect relative movement between the adjoining components. Movement between adjoining components should have manifested into signs of fretting on their mating surfaces, but visual microscopic examinations carried out at the Engineering Branch did not identify any signs of fretting on any of the mating surfaces nor on any of the ten screws. Since the compressor rotor assembly was balanced prior to installation in the engine and subsequent vibration checks did not identify anomalies, it is concluded that the crushed piece of material found between the first and second compressor disks did not affect the final dimension or alignment of the rotor assembly. The ovalized shape is indicative of the rotor assembly rotating in an unbalanced condition. Each time contact occurred, the centre of mass of the rotor changed, resulting in rotation about a new mass centreline, exacerbating the situation. No conclusion could be reached with respect to the mode of failure of the smeared blade fracture surfaces. It is possible that a progressive or fatigue type of failure may have existed, but none was found. The summary of all information gathered leads to the conclusion that, for undetermined reasons, an imbalance developed within the compressor rotor during engine operation. This resulted in a wobble and subsequent contact between the rotor assembly and the surrounding stationary components. In combination, any disturbance to the airflow and any friction slowing the compressor rotor resulted in reduced airflow to the combustion chamber, thereby enriching the fuel to air mixture and increasing the temperature at the CT nozzle and disk. A loss of compressor efficiency resulted in a loss of N2(PT and main rotor speed), which triggered the PTG to increase the fuel flow (increase the compressor speed toward the topping limit) to maintain the N2at the selected speed (100percent), thus further increasing the temperature at the CT nozzle vanes and CT disk blades to the melting point. If the compressor was operating at N1speeds in excess of the placard limit, this process would likely have been accelerated. In addition, compressor blade tip clearance may also have been reduced, thus increasing the risk of contact between rotating and stationary components. When the blade contact became severe enough, fourth- or fifth-stage compressor blades began to break due to overload, resulting in the destruction of all fourth- and fifth-stage blades. This resulted in the total failure of the engine before the PT nozzle and blades began to melt. 2.3 External Load Operations The electrical function of the remote hook on the longline was intentionally disabled; therefore, the bucket could not have been released from the longline by the pilot. Discussion and demonstrations show that it is possible to twist a shackle out of the hook and it was concluded that, since there was no damage to the remote hook or the bucket control head and shackle, this was the most likely explanation for the detachment of the bucket when it contacted the trees. The location of the external cargo release switch varies on different helicopters and with different operators. This results in a situation where a pilot, when moving from one helicopter or operator to another, is not accustomed to the switch location. In emergency situations, a pilot may not be able to quickly and instinctively activate the release switch. In the accident helicopter, the switch position on the cyclic control grip was not what the pilot was accustomed to. It is highly likely that the pilot had yet to change his automatic behaviour and activate the external cargo release switch in the new location in an emergency. Therefore, it is probable that the pilot's action during the emergency did not activate the external cargo hook release mechanism and, rather, that the trailing longline snagged a tree while the helicopter was still airborne. This factor was an additional complication to the survivability aspects of this accident; it could not be speculated whether items such as the pilot's safety harness or seat, or the aircraft's vulnerability to impact forces or post-impact fire would have permitted the pilot to survive the impact. The designation of a specified switch location for the external cargo release is not required by regulation; hence, many variations present a risk of unsuccessful or inadvertent release. A backup quick-release method is also required by regulation. The foot pedal release installed in the accident helicopter was an approved system; however, its effectiveness is reduced because it requires the pilot to take one foot off of a primary flight control in an emergency. In TSB investigation A93W0159, it was determined that the pilot could not operate the foot pedal due to the uncontrolled behaviour of the helicopter. The following TSB Engineering Laboratory report was completed: This report is available upon request from the Transportation Safety Board of Canada. 3.0 Conclusions 3.1 Findings as to Causes and Contributing Factors An imbalance of the engine compressor rotor assembly developed during the operation of the engine, resulting in contact between the rotor and stator assemblies. The contact led to the destruction of the compressor rotor assembly and engine failure. No conclusion could be reached with respect to the mode of failure that caused the imbalance. The combination of altitude, terrain features and the trailing longline negatively affected the pilot's ability to complete a successful emergency landing in autorotation. 3.2 Findings as to Risk Some procedures used in the engine overhaul process were not in accordance with the manufacturer's overhaul manual; failure to comply with the manufacturer's instructions could compromise the integrity of the assembly and result in failure. Field adjustments to the engine fuel control take-off trim without the confirmation of an N1 topping check for accuracy introduces a risk of frequent or continuous operations at gas generator speeds and internal temperatures beyond established limits. An inconsistent placement of the external cargo release switch increases the risk of pilot confusion during an emergency when trying to activate the external cargo hook-release mechanism, possibly complicating an emergency landing. The foot pedal backup quick release is an approved system. However, its effectiveness is reduced because it requires the pilot to take one foot off of a primary flight control in an emergency. On 09December2003, a Federal Aviation Administration (FAA) inspection of Cappsco International facilities and procedures was conducted. It was determined by the FAA that, at the time of the inspection, the inspectors are confident that Cappsco International has the data, experience and knowledge to properly overhaul the engines for which they are rated. All Canadian operators of the T5311Bengine were advised of the safety concerns identified during the overhaul process at Cappsco International facilities. Gemini Helicopters Inc. has standardized the cyclic grips in all of its aircraft (excluding the Robinson44s, which are incompatible for such a modification) so that the switches are the same in each type. It has also put the emergency (manual) release on the collective in its EurocopterAS350s and is searching for Supplemental Type Certificates applicable to the rest of its fleet. The rationale is that the emergency-release systems (isolated pull handles or foot pedals) in the other aircraft also require the use of either hands or feet to operate; therefore, requiring the pilot to let go of a flight control to release an external load via the manual release. With the manual release on the collective, activation is possible without requiring pilots to remove their hands or feet from primary flight controls.4.0 Safety Action Taken On 09December2003, a Federal Aviation Administration (FAA) inspection of Cappsco International facilities and procedures was conducted. It was determined by the FAA that, at the time of the inspection, the inspectors are confident that Cappsco International has the data, experience and knowledge to properly overhaul the engines for which they are rated. All Canadian operators of the T5311Bengine were advised of the safety concerns identified during the overhaul process at Cappsco International facilities. Gemini Helicopters Inc. has standardized the cyclic grips in all of its aircraft (excluding the Robinson44s, which are incompatible for such a modification) so that the switches are the same in each type. It has also put the emergency (manual) release on the collective in its EurocopterAS350s and is searching for Supplemental Type Certificates applicable to the rest of its fleet. The rationale is that the emergency-release systems (isolated pull handles or foot pedals) in the other aircraft also require the use of either hands or feet to operate; therefore, requiring the pilot to let go of a flight control to release an external load via the manual release. With the manual release on the collective, activation is possible without requiring pilots to remove their hands or feet from primary flight controls.